This invention relates to ultrasonic depth indicators and in particular, to a new and improved depth indicator particularly suited for locating fish. The ultrasonic depth indicators presently in use have an ultrasonic transducer carried on a boat for transmitting and receiving pressure waves in the water. An electrical transmitter provides an electrical pulse to the transducer which in turn produces the pressure wave in the water. This pressure wave is reflected by various objects in the water and these echoes are received by the transducer and converted to electrical signals directed to a receiver.
The depth indicator includes a display having a rotating member, typically a wheel carrying a lamp, and the receiver output is used to energize the lamp, causing the lamp to flash at predetermined locations in a revolution of the wheel. The display may be marked off in feet or fathoms or otherwise as desired, providing a visual display of the depth at which the echo is produced. Instruments of this type are in general use today.
However there are disadvantages with the current equipment which causes ambiguous displays and masks desirable information. Where the time interval for the travel of the ultrasonic pressure pulse from the transducer to the bottom and return is greater than the time interval between pulses, the echo from the first pulse will be displayed after the transmission of the second pulse and will appear to be from a much shallower depth than actually is the fact. For example, if the depth indicator indicates depth from 0 to 60 feet, an echo from an 80 foot bottom will be indicated as being from a 20 foot bottom.
Also, where the bottom is a good reflector, the transmitted pulse can bounce back and forth between the bottom of the body of water and the surface of the water or the bottom of the boat one or more times providing a plurality of indications at multiples of the actual depth. For example, if the depth is 25 feet, displays will be provided at 25 feet, 50 feet and 75 feet. With the depth indicator previously mentioned having the 0 to 60 foot display, these three echoes would produce displays at 25 feet, 50 feet and 15 feet.
These modes of operation are undesirable because they give erroneous indications of bottom depth. Also, the displays will mask other echoes such as those produced by fish. For example, when operating with the 60 foot indicator in 80 feet of water, the echo from the bottom will mask echoes from fish at about 20 feet. Similarly, when receiving multiple echoes from a 25 foot bottom, fish at 15 feet will be masked. In the past, multiple echoes have been reduced by reducing the gain of the system. However this is undesirable because echoes from fish normally are of lower energy than echoes from the bottom and hence reducing gain causes a loss of the fish echo.
It is an object of the present invention to provide a new and improved depth indicator which can eliminate the ambiguities and errors discussed above while permitting operation at full gain so that fish echoes are not lost nor masked.